Obviously the two families are very different and are juxtaposed to show dramatic contrast. In particular, consider the following differences and how they enhance the contrast between the Macduffs and the Macbeths.

Firstly, Macbeth shows himself to be willing to commit murder to secure his own rise to power and then to protect his position. Macduff, as we see in Act IV scene 3, is fiercely loyal to Scotland above all, and will do anything to protect it from the exploitation of characters like Macbeth. Note how he addresses Scotland whilst talking to Malcolm:

Bleed, bleed, poor country!

Great tyrany, lay thou thy basis sure,

For goodness dare not check thee! wear thou thy wrongs;

Thy title is affeer'd!

Macduff is clearly the polar opposite of Macbeth, who will do anything to seize and maintain his power.

Secondly, Lady Macduff and Lady Macbeth are polar opposites as well. Lady Macduff only appears in one scene, Act IV scene 2, but her role as a mother figure is reinforced by her playful conversation with her eldest son. She, like her husband, is presented as a likeable, loving figure, who is passionately committed to family. Lady Macbeth, by contrast, has no family and is all to willing to sacrifice her "sex" when she implores the spirits to make her less human and more evil to ensure her husband's rise to power.

Thus, all in all, we can see how these two families are deliberately contrasted and are completely different. Of course, the slaughter of the Macduff family is yet another indicator of Macbeth's descent into evil, and the conflict between Macduff and Macbeth seems to represent the battle between good and evil.

The Macduffs serve as foils to the Macbeths. Both Macbeth and Macduff are nobles in Duncan's Scottish kingdom, and both are strong, talented soldiers.

Whereas Macbeth allows his desire for power to lead to his downfall, Macduff remains loyal to the rightful king (Duncan, followed by Malcolm).

Whereas Lady Macbeth does not seem to have children (Experts have debated her comment, "I have suckled ...), Lady Macduff has a number of children and is clearly a loving, devoted mother. Lady Macbeth not only urges, but browbeats her husband to throw away his honor and commit murder, killing the king who had just heaped a new title (with the associated castle and lands) on him. Lady Macduff criticizes her husband (while speaking to her son) because she believes he has failed to uphold his honor.

Macbeth -- goes from the best of men to one who has sold his soul to the devil.

Macduff -- a loyal subject who fights to restore the throne to the rightful person, even though it costs him his wife and children.

Lady Macbeth -- think golddigger, who wants everything, no matter the cost, and eventually goes mad from guilt.

Lady Macduff -- a great mother, who loves her family and dies trying to protect them.